Effect of hypothermia on cell kinetics and response to hyperthermia and X rays

Hyperthermia is a potent radio enhancer. Studies using hypothermia in combination with irradiation have given confusing results due to lack of uniformity in experimental design. This report shows that hypothermia might have potential significance in the treatment of malignant cells with both thermo- and radiotherapy. Reuber H35 hepatoma cells, clone KRC-7 were used to study the effect of hypothermia on cell kinetics and subsequent response to hyperthermia and/or X rays. Cells were incubated at 8.5 degrees C or between 25 and 37 degrees C for 24 hr prior to hyperthermia or irradiation. Hypothermia caused sensitization to both hyperthermia and X rays. Maximum sensitization was observed between 25 and 30 degrees C and no sensitization was found at 8.5 degrees C. At 25 degrees C maximum sensitization was achieved in approximately 24 hr, cell proliferation was almost completely blocked, and cells gradually accumulated in the G2 phase of the cell cycle. In contrast to the effect of hypothermia on either hyperthermia or X rays alone, thermal radiosensitization was decreased in hypothermically pretreated cells (24 hr at 25 degrees C) compared to control cells (37 degrees C). The expression of thermotolerance and the rate of development at 37 degrees C after an initial heating at 42.5 degrees C were not influenced after preincubation at 25 degrees C for 24 hr. The expression of thermotolerance for heat or heat plus X rays during incubation at 41 degrees C occurred in a significantly smaller number of cells after 24 hr preincubation at 25 degrees C. The enhanced thermo- and radiosensitivity in hypothermically treated cells disappeared in approximately 6 hr after return to 37 degrees C.